Alane (also called aluminum hydride, with the chemical formula AlH3) is a potential source of hydrogen for future fuel cell powered vehicles. Onboard a fuel cell vehicle, alane can be decomposed to give hydrogen. A byproduct of the reaction is aluminum metal. For alane to be widely used in fuel cell vehicles, the aluminum metal must be reprocessed back into alane with high energy-efficiency. Directly reacting aluminum metal and hydrogen gas to produce alane is difficult because the thermodynamics are not favorable.
The synthesis of alane is well developed. Beginning in the 1960's (and continuing today) alane has been considered an attractive rocket propellant. However, thus far there has been no need to directly react aluminum and hydrogen to form alane. Therefore, because directly reacting aluminum and hydrogen is difficult, the prior art synthesis procedures are indirect. For example, the best developed synthesis of alane (AlH3) begins with aluminum chloride (AlCl3) and sodium alanate (NaAlH4). These compounds are reacted in a solvent, such as tetrahydrofuran (THF) according to the reaction3NaAlH4+AlCl3→4AlH3+3NaCl  Reaction 1which gives alane and the byproduct NaCl. For this synthesis method to be used to reprocess aluminum, the aluminum together with the NaCl generated in Reaction 1, must first be processed into AlCl3 and NaAlH4. These reactions can be carried out by established methods but are energetically very inefficient.
The thermodynamics of alane have also been well studied. These studies indicate that the direct synthesis of alane from aluminum and hydrogen, proceeds according to the reactionAl+3/2H2→AlH3  Reaction 2
Using the thermodynamic calculation module in HSC Chemistry for Windows, the standard enthalpy change, ΔH°, for the direct formation of alane from aluminum metal and hydrogen gas according to Reaction 2 is −11.3 kJ/mol-AlH3 or −7.5 kJ/mol-H2. Because ΔH° is negative, this reaction is exothermic and might be expected to proceed spontaneously. However, because hydrogen gas is being incorporated into a solid phase, the standard entropy change is also negative. From HSC, ΔS°=−194.8 kJ/K-mol-AlH3 or −129.9 kJ/K-mol-H2. Thus, the standard Gibb's free energy change, ΔG°, which is given byΔG°=ΔH°−T*ΔS°  Equation 1where T is the absolute temperature, is +45.5 kJ/mol-AlH3 or +30.3 kJ/mol-H2 at 20° C. (293 K). Because ΔG° must be negative for a reaction to proceed, the direct synthesis of alane, according to Reaction 2, does not occur under standard conditions. Reaction 2 can be forced to proceed by increasing the pressure until the loss of entropy is overcome. The positive ΔG° may be overcome by applying very high pressures on the order of 104 to 105 atmospheres. However, using these high pressures is very energetically inefficient, technologically difficult and not practical. Because of these limitations, direct synthesis at high pressures has not been widely practiced.
There are other problems associated with the synthesis and storage of alane. Alane decomposes in water. Further, alane decomposes at temperatures above approximately 100° C.